Research On Key Points Of Health Monitoring Of Through Type Concrete Filled Steel Tube Tied Arch Bridge In Plateau Valley

Concrete filled steel tubular tied arch bridges have been extensively constructed nationwide since they developed due to their superior mechanical and construction performance.As a multiple statically indeterminate complex structure composed of a beam arch structure,it uses a concrete filled steel tube composite structure as the main load-bearing component.During operation,various diseases and even engineering accidents are prone to occur at the arch ribs,suspenders,and tie bars of the structure under external action.Therefore,it is very important to monitor the health of the structure,grasp the operational status of the structure in real time,and set a safety baseline during the operation of the structure.This article takes the through steel tube concrete tied arch bridge located in the plateau valley region as the research object,and the research covers the structural construction and operation period.Based on the construction materials and structural characteristics of the bridge,combined with the topographic characteristics of the bridge site,the key points of health monitoring are studied.The main research contents and conclusions are as follows:(1)In order to make the established finite element model more reasonable,a model considering the geometric initial error of the arch axis and the initial stress of the steel pipe was established.The calculation results show that the geometric initial error of the arch axis and the initial stress of the steel pipe have a significant impact on the stress state of the structure during the operation of the completed bridge and it is necessary to consider the geometric initial error of the arch axis and the initial stress of the steel pipe during establishing a finite element model.The geometric alignment of the arch rib should be considered as key points for structural health monitoring.At the same time,health monitoring should be conducted on the stress of the quarter section of the upper chord steel pipe and the section of the lower chord steel pipe arch foot to ensure that the maximum value of the section stress does not exceed the allowable range.Finally,the finite element model is modified based on the actual construction monitoring data,and a more realistic finite element model is obtained.(2)The calculation methods and effects of CFST shrinkage and creep,temperature effects,and static wind loads for the Mugao Jinshajiang River Bridge are analyzed.The direct calculation method+equivalent temperature reduction method,and direct calculation method are recommended to calculate the shrinkage and creep displacement and stress of CFST respectively;The uniform temperature effect and gradient temperature effect of the system have a significant impact on the displacement,internal force,and stress response of the bridge structure.It is recommended to use the actual sunlight temperature field to calculate the temperature gradient curve of CFST dumbbell arch rib section in the plateau valley region;Under static wind load,the quarter section of the arch rib also has a significant response to the lateral displacement at the arch crown and the lateral bending moment at the arch foot.Therefore,environmental and structural temperature as well as wind load related parameters should be taken as key points for health monitoring.At the same time,the calculation results of static and dynamic characteristics of the structure during the operation stage are analyzed,and the key points for structural deformation,internal force,and stress health monitoring are proposed based on the analysis results.(3)The effects of suspender fracture and tie steel strand fracture on the structure are discussed respectively.At the same time,a detailed analysis of the arch foot is conducted using solid elements to study the diagonal section crack resistance performance of the arch foot concrete during structural operation,especially under the action of temperature.The results show that the fracture of a single or double suspender has a significant impact on the internal force of the suspender,the stress of the arch rib and the tie bar;The fracture of single,double,and three steel strands will have a certain impact on the compressive stress of the main arch,and will seriously affect the internal force distribution of the tie bar on the side of the steel strand fracture,endangering the safety state of the structure;During the normal operation of structure,the contact position between the dumbbell shaped arch rib and the rectangular section of the arch foot concrete and the left section of the contact position,as well as the intersection of the tie bar concrete and the arch seat concrete,have different degrees of dangerous areas that exceed the stress specified in the specifications.There is a risk of cracks in the concrete in these areas,so it is necessary to pay attention to the above parts,especially when the system temperature changes,the dangerous areas where cracks are prone to occur in the concrete at the arch foot are increasing.Therefore,when the climate is close to extreme temperatures during structural operation,attention should be paid to the dangerous areas where cracks are prone to occur in the concrete surrounding the arch foot.At the same time,health monitoring methods and monitoring focuses for arch rib void are proposed.(4)Using the parameter sensitivity analysis method,the sensitivity of structural response to wind speed,structural temperature,arch rib inclination angle,and arch abutment angle parameters during operation of a through type concrete filled steel tubular tied arch bridge is discussed.The results show that wind speed changes have a significant impact on the lateral displacement and bending moment of the arch rib;The vertical displacement,internal force,and stress of arch ribs are sensitive to temperature changes;The change of arch rib inclination angle has a certain impact on the maximum stress of the steel pipe;The bending moment at the arch foot and tie rod end is very sensitive to the slight rotation of the arch base,and also has a certain impact on the maximum stress of the steel pipe.Finally,the health monitoring content and methods of concrete filled steel tubular tied arch bridges are determined based on the previous content,and a health monitoring scheme for concrete filled steel tubular arch bridges is proposed.At the same time,the structural safety early warning threshold and safety early warning classification are determined.The corresponding safety early warning thresholds for different early warning levels are calculated using arch rib deflection,beam deflection,and suspender cable force as examples.